A facsimile machine that selects the optimum transmission speed in conformity with the transmission quality of a communication line has been proposed, as disclosed in the specification of Japanese Patent Application Laid-Open No. 5-252370.
Also available is an image scanner that uses an interface compliant with RS 232C, IEEE 1284 (parallel), SCSI or USB 1.1, or a special-purpose interface, in order to be connected to a personal computer.
A bottleneck encountered in terms of image reading speed in these conventional image sensing apparatus is the speed of the interface. In an image sensing apparatus that uses an interface having a comparatively low speed, however, high-quality, efficient transmission is carried out by converting image data to eight bits and transmitting the 8-bit data after image processing such as a shading correction, gamma correction and resolution conversion is executed by hardware.
High-speed serial interfaces such as those compliant with USB 2.0 and IEEE 1394 have been standardized in recent years, thereby making possible an environment in which high-speed transmission can be achieved.
Further, proposal for widening a variable magnification range by changing speed reduction ratio using an electromagnetic switch in accordance with a scanning resolution is disclosed in Japanese Patent Application Laid-Open No. 5-300333.
Furthermore, an apparatus that improves driving efficiency by transmitting driving power of driving means to operation means by using different transmission gear ratios for forward scan and backward scan changed by a single direction clutch as well as reduces generation of vibration at the time of image scanning is suggested in Japanese Patent Application Laid-Open No. 2000-013574.
Further, as an image reading apparatus connected to an external apparatus such as a computer or the like when it is used, an image reading apparatus, which operates using electric power supplied from a power supply of the external apparatus via a connection cable such as a USB cable or the like, or using electric power supplied from an AC power supply, is known.
However, when the speed of a processing circuit is raised in order to raise reading speed and exploit a high-speed interface effectively, the cost of such circuitry as an image processing circuit and memory rises and there is also an increase in power consumption and unnecessary radiation.
On the other hand, in a processing circuit adapted to operate with a low-speed interface, the high-speed nature of an interface does not manifest itself sufficiently. For example, if a 2-MB buffer memory is constituted by a DRAM, the fact that the DRAM used customarily has a cycle time of 100 ns at the time of random access means that memory access will represent a bottleneck with regard to reading speed when image processing that makes repeated reference to the memory is executed.
Further, minimizing image processing that refers to a memory, such as processing for a shading correction or gamma correction, to thereby reduce the number of times memory is accessed also is effective in outputting image data at high speed. For example, if an analog/digital converter is a 14-bit converter, one available method is to transmit 14-bit raw data to a host computer as is and execute all image processing such as a shading correction using the memory and arithmetic and logical circuitry of the host computer.
In the case of an image sensing apparatus thus constructed, the amount of data is nearly doubled because 14 bits are transmitted. However, merely a high-speed, small buffer memory will suffice because the transmission speed of image data is sufficiently high in comparison with the speed at which image data is generated. However, in a case where a hardware implementation is thus adopted in conformity with the high-speed transmission mode, 14-bit data is transmitted in order to avoid a decline in image quality at such times that it is necessary to use a low-speed transmission mode because of the configuration of the image sensing system. When it is thus attempted to transmit 14-bit data, the total amount of data not only nearly doubles in comparison with the transmission of 8-bit data after image processing but the speed of data generation also increases in comparison with the data transmission speed. As a consequence, the buffer memory soon is filled to capacity, the reading of the image data must be suspended and resumed repeatedly and it is likely that reading speed will diminish to a great degree.
Further, a transmission gear mechanism has not been controlled in accordance with the communication speeds of interfaces conventionally, and thus the apparatus can not sufficiently cope with a wide communication speed range. This may result in a large vibration when operating at low speed and large energy consumption when operating at high speed.
Further, in an image reading apparatus utilized by connecting to an external apparatus such as a computer, when such image reading apparatus operates using electric power supplied from a power supply of an external apparatus, it may fail to operate normally due to insufficient electric power. This problem readily occurs when the external apparatus is driven using a battery.